Sydney has experienced its hottest day on record, with temperatures passing 45 degrees across the greater city area.
The mercury hit 45.8 degrees Celsius at Observatory Hill at 2.55pm, 0.5 above Sydney's previous hottest day in 1939.
Across greater Sydney, the hottest temperatures recorded were at Penrith (46.5C at 2.16pm), Camden (46.4C at 3.04pm), Richmond (46.4C at 3.01pm), Sydney Airport (46.4C at 2.32pm) and Bankstown (46.1C at 3.28pm).
Julie Evans from the weather bureau says a cool change will come through Sydney in the early evening.
"The good news is that the change will arrive this evening. It won't be after midnight like it was last time," she said.
"It's looking like it will arrive in the early to mid-evening, a bit later in the western suburbs."
By 4.15pm, the temperature at Observatory Hill, where the weather bureau started recording temperatures in 1859, had dropped to 35C.
There are major delays to trains across the CityRail network due to overhead wiring and power supply problems due to the heat.
CityRail is asking passengers to take care in the extreme heat and travel with a water bottle.
It says train tickets can be used to catch Sydney Buses services and Sydney Ferries services.
The race meeting at Gosford has also been called off as the mercury there passed 44C.
Canberra, meanwhile, has broken the record for its hottest January day with temperatures reaching 41.6C, breaking the previous record of 41.4C.
Paramedics feel the heat
The NSW Ambulance Service has been flooded with calls for people seeking help for heat exposure.
Paramedics say they have treated 52 people for heat stroke, with 100 more treated for fainting and around 40 for heat sickness.
Chief Superintendent Ian Johns says that figure is just the tip of the iceberg.
"We get hundreds more calls for people complaining of heat-related illness," he said.
"People call 000 for dizziness or vomiting or headache or generally unwell or collapse, and when the paramedics get there and do some assessment it can be attributed to the excessive heat.
"People underestimate the heat and overestimate their ability and that would be particularly so for younger, fitter Australians."
Elderly people and those with chronic illness need to be especially careful.
Mr Johns has advised people to take simple measures such as wearing a hat, carrying water and staying out of the heat.
"The better you can prepare yourself, the better chance you have of getting through these extreme weather days with your health intact."
© ABC 2013
12:38 EST Tropical Cyclone Jack has rapidly developed over the warm waters of the Indian Ocean in the past 24 hours, making it the 10th system to be named this season.